US7361507B1 - Method for modulating nitrilase selectivity, nitrilases obtained by said method and use thereof - Google Patents

Method for modulating nitrilase selectivity, nitrilases obtained by said method and use thereof Download PDF

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US7361507B1
US7361507B1 US10/129,560 US12956000A US7361507B1 US 7361507 B1 US7361507 B1 US 7361507B1 US 12956000 A US12956000 A US 12956000A US 7361507 B1 US7361507 B1 US 7361507B1
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nitrilase
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Jérôme Pierrard
Olivier Favre-Bulle
Catherine Jourdat
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Adisseo France SAS
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    • C12N9/00—Enzymes; Proenzymes; Compositions thereof; Processes for preparing, activating, inhibiting, separating or purifying enzymes
    • C12N9/14—Hydrolases (3)
    • C12N9/78—Hydrolases (3) acting on carbon to nitrogen bonds other than peptide bonds (3.5)
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y10T436/14—Heterocyclic carbon compound [i.e., O, S, N, Se, Te, as only ring hetero atom]
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    • Y10T436/143333—Saccharide [e.g., DNA, etc.]

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  • the present invention relates to novel nitrilases with enhanced selectivity, to the method for obtaining them and to the use of said nitrilases.
  • FIGS. 1A-E show an alignment of the amino acid sequences of nitrilases from several species.
  • the aligned amino acid sequences are p_Athalia1 (SEQ ID NO: 6), p_Athalia2 (SEQ ID NO: 7), p_Athalia3 (SEQ ID NO: 8), p_Tobacco1 (SEQ ID NO: 9), p_Tobacco2 (SEQ ID NO: 10), p_Osativa (SEQ ID NO: 11), p_Athalia4 (SEQ ID NO: 12), b_RhodocJ1 (SEQ ID NO: 13), b_RrhodocPA3 (SEQ ID NO: 14), b_Gterrae (SEQ ID NO: 15), b RrhodocK22 (SEQ ID NO: 16), b Kozaenae (SEQ ID NO: 17), b_CtestosNI1 (NitA; SEQ ID NO: 18), b_Afaecalis
  • each sequence panel represents the amino acid position of the NitB (b_Afaecalis of Alcaligenes faecalis ) reference sequence (SEQ ID NO: 19).
  • the “**” indicator designates the amino acids of each sequence corresponding to positions 162 and 163 of NitB.
  • FIG. 2 shows a map of plasmid pRPA-BCAT41
  • Enzymes which catalyze the hydrolysis of nitrile groups to corresponding carboxylic acids and ammonium ions are nitrilases (Faber, Biotransformations in Organic Chemistry, Springer Verlag, Berlin Heidelberg, 1992, ISBN3-540-55762-8).
  • this bioconversion of the nitrile groups to corresponding carboxylic acids, the final result of which consists of hydrolysis of the nitrile groups may also be carried out in two steps, the first step consisting of the bioconversion of the nitrites to corresponding amides with a nitrile hydratase, the second step consisting in hydrolyzing the amides obtained to corresponding carboxylic acids with amidases.
  • Nitrilases were first discovered in plants (Thimann and Mahadevan, 1964 , Arch. Biochem. Biophys. 105: 133-141) and then isolated in many representatives of soil microflora (Kobayashi and Shimizu, 1994 , FEMS Microbiology Letters 120: 217-224): Pseudomonas, Nocardia, Arthrobacter, Fusarium, Rhodoccocus, Klebsiella and Alcaligenes . More recently, nitrilases have been characterized in thermophilic bacteria (Cramp et al., 1997 , Microbiology, 143: 2313-2320).
  • Nitrilases have varied substrate specificities but can be grouped into three groups depending on their specificity: nitrilases specific for aliphatic nitriles, those specific for aromatic nitrites or those specific for arylacetonitriles (Kobayashi et al., 1993 , Proc. Natl. Acad. Sci. USA 90: 247-251; Kobayashi and Shimizu, 1994, mentioned above; Lévy-Schil et al., 1995 , Gene 161: 15-20; Layh et al., 1998, J. Mol. Catal. B: Enzymatic 5: 467-474).
  • Nitrilases are of value in biocatalysis since many synthetic processes involve the hydrolysis of nitrile groups (Yamamoto et al., 1991, Appl. Environ. Microb. 57: 3028-3032; Faber, Biotransformations in Organic Chemistry, 2nd edn, Springer-Verlag, Berlin, 1995; Lévy-Schil et al., 1995 , Gene 161: 15-20; Cowan et al., 1998, Extremophiles 2: 207-216): conversion of adiponitrile to cyanovalerate or adipate, synthesis of nicotinic acid, of p-aminobenzoic acid of tranexamic acid, enantioselective hydrolysis of mandelonitrile.
  • NitB the nitrilase of Alcaligenes faecalis ATCC8750
  • NitA Comamonas testosteroni
  • Nitrilases have primary structures which align with variable degrees of identity, starting from approximately 30%. Aligning the sequences of several nitrilases reveals the conservation of several residues, including a cysteine residue at position 163 on the sequence of the NitB nitrilase. This residue is involved in the nitrilase reaction mechanism (Kobayashi et al., 1993, Proc. Natl. Acad. Sci. USA 90: 247-251).
  • the reference sequence is the NitB sequence, all the definitions and indications of particular amino acid positions being given relative to the NitB primary sequence.
  • FIG. 1 represents an alignment of 14 nitrilase sequences described in the state of the art, aligned relative to the NitB sequence as reference, comprising the sequences p_Athalia1 to 4 of Arabidopsis thaliana (SwissProt accession No.: P32961, P32962, P46010, P46011), p_Tobacco1 and 2 of Nicotiana tabacum (GeneBank accession No.: D63331, D83078), b_RhodocJ1 of Rhodococcus rhodocrous J1 (GeneBank accession No.: D11425), b_RrhodocPA3 of Rhodococcus rhodocrous PA34 (GeneBank accession No.: E09026), b_Gterrae of Gordona terrae (Genebank accession No.
  • the numbering of the amino acids of the NitB sequence is given on this figure (numbering at the bottom), as is the consensus sequence with its numbering (numbering at the top).
  • the residues of the other nitrilases are numbered relative to this cysteine residue and to the sequence of the NitB nitrilase as reference sequence. Based on such an alignment, or on any nitrilase sequence of alignment, it is easy for those skilled in the art to identify, using the definition of the NitB amino acid given by its position and its nature, the position of the corresponding amino acid in another nitrilase sequence.
  • Nitrilase selectivity is defined as the percentage of compounds not having a carboxylic function which are released by the nitrilase-catalyzed hydrolysis of a nitrile. It has been described relatively little, but is observed for a certain number of nitrilases and substrates. Thus, the hydrolysis of 2-methoxymandelonitrile to 2-methoxymandelic acid catalyzed by the nitrilase of Pseudomonas fluorescens DSM 7155 leads to the coproduction of 2-methoxy-mandelamide (Layh et al., 1998, mentioned above).
  • HMTBN 2-hydroxy-4-methylthio-butyronitrile
  • HMTBM 2-hydroxy-4-methylthiobutyramide
  • An increase in enzyme selectivity may also be sought if this increase is accompanied by an increase in the catalytic activity of the enzyme on its substrate. It is in particular the case in methods of decontamination in which rapid degradation of a toxic molecule with an enzyme with maximum specific activity is sought. In this case, the nature of the products derived from the reaction catalyzed by the enzyme has little importance relative to the rate of degradation of the substrate.
  • the directed evolution of an enzyme consists in adapting an enzyme to a particular function by repeatedly selecting variants which have enhanced properties (Arnold and Volkov, 1999, Current Opinion in Chemical Biology 3: 54-59; Kuchner and Arnold, 1997, Tibtech 15: 523-530). These variants may be created by several techniques of mutagenesis on the gene encoding the enzyme studied (Skandalis et al., 1997, Chemistry & Biology 4: 8889-898; Crameri et al., 1998, Nature 391: 288-291): chemical mutagenesis (Singer and Fraenkel-Conrat, 1969, Prog. Nucl. Acid Res. Mol. Biol.
  • the present invention therefore relates to a modified nitrilase with modulated selectivity, characterized in that it comprises, at position 162, an amino acid residue which is different from the amino acid residue of origin.
  • modified nitrilase is intended to mean a nitrilase which is modified relative to a nitrilase of origin, the modification consisting in replacing the amino acid residue of origin at position 162 with another amino acid.
  • the expression “modulation of the selectivity” is intended to mean a selectivity of the modified nitrilase which is different from the selectivity of the nitrilase of origin, in particular by at least 0.5% relative to the nitrilase of origin, advantageously by at least 1%.
  • residue 162 is replaced with an amino acid chosen from cysteine, alanine, valine, asparagine, glutamine, isoleucine and serine, it being understood that residue 162 of the nitrilase of origin is different from a cysteine, alanine, valine, asparagine, glutamine, isoleucine or serine, respectively.
  • Residue 162 is preferably replaced with a cysteine residue.
  • the modulation consists of an enhancement of the selectivity.
  • the modulation consists of a decrease in the selectivity.
  • the unmodified nitrilase of origin is chosen from nitrilases of bacterial, yeast, fungal, plant or animal origin.
  • nitrilases of bacterial origin mention may be made, in particular, of the following nitrilases: b_RhodocJ1 of Rhodococcus rhodocrous J1 (GeneBank accession No.: D11425), b_RrhodocPA3 of Rhodococcus rhodocrous PA34 (GeneBank accession No.: E09026), b_Gterrae of Gordona terrae (GeneBank accession No.: E12616), b_RrhodocK22 of Rhodococcus rhodocrous K22 (GeneBank accession No.: D12583), b_Kozaenae of Klebsiella ozaenae (SwissProt: accession No.: P100450), b_CtestosNI1 of Comamonas testosteroni NI1 or NitA (GeneBank accession No.: L32589), and b_Afaecalis of
  • p_Athalia1 to 4 of Arabidopsis thaliana (SwissProt accession No.: P32961, P32962, P46010, P46011), and p_Tobacco1 and 2 of Nicotiana tabacum (GeneBank accession No.: D63331, D83078).
  • nitrilases of other origins mention may be made in particular of: those of Saccharomyces cerevisiae (SwissProt accession No.: P40447 and P4044), of Caenorhabditis elegans (GeneBank accession No.: AF069986), of Drosophila melanogaster (GeneBank AF069989) of Homo sapiens (GeneBank accession No.: AF069987) and of Mus musculus (GeneBank accession No.: AF069988).
  • the nitrilase of origin is a nitrilase obtained by screening DNA libraries, in particular cDNA or genomic DNA from various sources, in particular DNA libraries obtained through random mutations and recombinations of nitrilases, by directed molecular evolution, or by screening a DNA library from soil or from other biotopes.
  • the present invention also relates to a nucleic acid sequence, in particular a DNA sequence, encoding a modified nitrilase above.
  • the nucleic acid sequence according to the invention consists of the nucleic acid sequence of the nitrilase of origin, for which the codon of the residue of origin at position 162 has been replaced with a codon encoding a residue which is different from the residue of origin, in particular the codons encoding the residues alanine, valine, asparagine, glutamine, isoleucine or serine.
  • codons of the sequence of origin may be modified by any means known to those skilled in the art for enhancing the enzymes defined previously, in particular by directed mutagenesis.
  • the present invention also relates to a chimeric gene or expression cassette, comprising, in the direction of transcription, a promoter regulatory sequence which is functional in a host organism, the nucleic acid sequence encoding a modified nitrilase according to the invention and a terminator regulatory sequence which is functional in the same host organism.
  • the host organism comprises any eukaryotic or prokaryotic organism, which may be differentiated or undifferentiated, in particular bacteria, yeasts, fungi, plant cells and plants.
  • bacteria for example E. coli
  • yeasts in particular of the genera Saccharomyces, Kluyveromyces or Pichia
  • fungi in particular of the genera Aspergillus or Penicillium
  • a baculovirus or plant cells and plants.
  • plant cell is intended to mean any cell which is derived from a plant and which can constitute undifferentiated tissues such as calluses, differentiated tissues such as embryos, parts of plants, plants or seeds.
  • the term “plant” is intended to mean any differentiated multicellular organism capable of photosynthesis, in particular monocotyledons or dicotyledons, more particularly crop plants which may or may not be intended for animal or human food, such as maize, wheat, rapeseed, soybean, rice, sugar cane, beetroot, tobacco, cotton, etc.
  • the promoter and terminator regulatory elements are well known to those skilled in the art, depending on the host organisms.
  • a regulatory sequence which is a promoter in bacteria use may be made of any promoter regulatory sequence of a gene expressed naturally in bacteria, for example the promoter of the E. coli tryptophan operon (Denèfle et al., 1987, Gene 56: 61-70).
  • a regulatory sequence which is a promoter in yeasts use may be made of any promoter regulatory sequence of a gene expressed naturally in yeasts, for example the promoter of the S. cerevisiae Mf ⁇ 1 gene or of the Kluyveromyces lactis lactase gene (van den Berg et al., 1990, Bio/Technology 8: 135-139).
  • a regulatory sequence which is a promoter in fungi use may be made of any promoter regulatory sequence of a gene expressed naturally in fungi, for example the promoter sequence of the Penicillium chrysogenum acid phosphatase gene (Graessle et al., 1997, Appl. Environ. Microbiol. 63:753-756) or the promoter sequence of the Aspergillus nidulans alcohol dehydrogenase I gene (Gwyne et al., 1989, Biochem. Soc. Trans. 17: 338-340).
  • a regulatory sequence which is a promoter in plant cells and plants use may be made of any promoter sequence of a gene expressed naturally in plants, in particular a promoter of bacterial, viral or plant origin, such as, for example, that of a gene of the ribulose-biscarboxylase/oxygenase (RuBisCO) small subunit, a histone promoter (EP 0 507 698), a rice actine promoter, or a promoter of a plant virus gene, such as, for example, that of the cauliflower mosaic virus (CAMV 19S or 35S), or a promoter inducible by pathogens, such as the tobacco PR-Ia, it being possible to use any suitable known promoter.
  • a promoter of bacterial, viral or plant origin such as, for example, that of a gene of the ribulose-biscarboxylase/oxygenase (RuBisCO) small subunit, a histone promoter (EP 0 507 698), a rice actine promoter, or
  • a regulatory sequence which is a terminator in bacteria use may be made of any terminator regulatory sequence of a gene expressed naturally in bacteria, for example the terminator regulatory sequence of the E. coli ribosomal RNA operon (Denèfle et al., 1987, Gene 56: 61-70).
  • PGK S. cerivisiae phosphoglycerate kinase
  • VPN van den Berg et al., 1990, Bio/Technology 8: 135-139
  • a regulatory sequence which is a terminator in fungi use may be made of any terminator regulatory sequence of a gene expressed naturally in fungi, for example the terminator regulatory sequence of the Trichoderma reesei pyruvate kinase gene (Schindler et al., 1993, Gene 130: 271-275).
  • any terminator regulatory sequence of a gene expressed naturally in plants for example the terminator of a gene of bacterial origin, such as for example the Agrobacterium tumefaciens nos terminator, of viral origin, such as for example the CaMV 35S terminator, or of plant origin, such as for example a histone terminator (EP 0 633 317).
  • the present invention also relates to a transformed host organism comprising a chimeric gene as defined above, in particular a host organism defined above into the genome of which the chimeric gene according to the invention has been stably integrated.
  • the present invention also relates to a method for producing modified nitrilases with reduced selectivity defined above, said method consisting in selectivity defined above, said method consisting in culturing the transformed host organism according to the invention and, where appropriate, in isolating the modified nitrilase, as a mixture or in a purified form.
  • the present invention also relates to the use of a modified nitrilase according to the invention, in a biocatalysis reaction in a method for synthesizing or degrading chemical compounds.
  • the present invention relates to a method for modulating nitrilase selectivity, said method comprising replacing residue 162, in a nitrilase of origin, with an amino acid residue which is different from the amino acid residue of origin.
  • residue 162 is replaced by introducing, into the nucleic acid sequence encoding the unmodified nitrilase of origin, a codon encoding a residue at position 162 which is different from the codon encoding the residue of the nitrilase of origin.
  • the nitrilase obtained using said modulation method is a nitrilase as defined above.
  • the nitrilase activity on 2-hydroxy-4-methyl-thiobutyronitrile is measured as follows:
  • a culture sample with a known optical density at 660 nm (OD 660 ) is taken and washed in 100 mM phosphate buffer, pH 7.0. Estimating the dry weight of the sample from the OD 660 (one OD 660 unit corresponds to a dry weight of 0.35 mg of dry cell/ml), approximately 1 mg of DC is taken up in 1 ml of 100 mM phosphate buffer, pH 7.0, and incubated in a closed 2-ml tube at 35° C. for 10 minutes. The kinetics are initiated by adding 17 ⁇ l of the solution of HMTBN at 78%, so as to achieve a concentration of 100 mM in the reaction mixture at the start of the assay. The reaction is incubated at 35° C. with stirring.
  • the eluent is composed of HPLC-quality acetonitrile diluted in 50 mM H 3 PO 4 (0.9 liters of 50 mM H 3 PO 4 mixed with 0.1 liter of acetonitrile). This eluent, which is filtered and degassed, percolates through the column with a flow rate of 1 ml/min and a pressure of 140 bar.
  • the column used is a 5 ⁇ m C18 Nucleosil column which is 250 mm in length and 4.6 mm in diameter (INTERCHIM. Ref. N5CC18-25F).
  • the volume injected is 5 ⁇ l and the detection is performed by reading the absorbence at a wavelength of 215 nm.
  • the HMTBM, HMTBS (2-hydroxy-4-methylthiobutanoic acid) and HMTBN (2-amino-4-methyl-thiobutanamide) peaks have respective retention times of 9 min, 11 min and 15.8 min.
  • the amounts of HMTBN, the HMTBM and the HMTBS are deduced from the measurement of the surface area of the peaks and comparison with the surface area of the peaks of a calibration mixture of known composition.
  • nitrilase activity on 2-amino-4-methyl-thiobutyronitrile is measured as follows:
  • a cell pellet (between 0.4 and 20 mg of DC) is resuspended in 200 mM borate buffer, pH 9.2, and incubated in a closed 2-ml tube at 30° C. for 10 minutes.
  • the kinetics are initiated by adding a solution of AMTBN in order to obtain a final concentration of 50 mM in the reaction mixture at the start of the assay.
  • the reaction is incubated at 30° C. with stirring. Every 15 minutes, a 50 ⁇ l sample of the suspension is withdrawn and mixed with 950 ⁇ l of the HPLC eluent (see composition below) to stop the reaction. After centrifugation, the supernatant is analyzed by HPLC as described below.
  • the eluent is composed of 1% of HPLC-quality acetonitrile diluted in a solution of H 3 PO 4 at 0.5% in water. This eluent, which is filtered and degassed, percolates through the column with a flow rate of 1 ml/min.
  • the column used is a 5 ⁇ m C18 Nucleosil column which is 250 mm in length and 4.6 mm in diameter (INTERCHIM. Ref. N5CC18-25F), maintained at a temperature of 40° C.
  • the volume injected is 20 ⁇ l and the detection is performed by reading the absorbence at a wavelength of 210 nm.
  • the AMTBM, AMTBS (2-amino-4-methylthiobutanoic acid) and AMTBN peaks have respective retention times of 4.0 min, 4.5 min and 5.0 min.
  • the amounts of AMTBN, the AMTBM and the AMTBS are deduced from the measurement of the surface area of the peaks and comparison with the surface area of the peaks of a calibration mixture of known composition.
  • FIG. 2 represents the map of plasmid pRPA-BCAT41. The sites in brackets are sites which have been eliminated during cloning.
  • Ptrp tryptophan promoter
  • nitB nitrilase gene
  • TrrnB transcription terminators
  • end ROP end of the gene encoding the ROP protein (Chambers et al., 1988, Gene 68: 139-149);
  • ORI origin of replication;
  • RNAI/II RNAs involved in replication (Chambers et al., mentioned above);
  • Tc tetracyclin resistance gene.
  • the 1.27 kb fragment containing the P trp promoter, the ribosome binding site of the ⁇ phage cII gene (RBScII) and the nitrilase gene of Alcaligenes faecalis ATCC8750 (nitB) was extracted from the plasmid pRPA6BCAT6 (Application FR 96/13077) using the EcoRI and XbaI restriction enzymes, in order to be cloned into the vector pXL642 (described in CIP application Ser. No. 08/194,588) opened with the same restriction enzymes.
  • pRPA-BCAT15 The resulting plasmid, pRPA-BCAT15, was opened with the StuI and BsmI enzymes and the 4.3 kb fragment was ligated with the 136 bp StuI-BsmI fragment purified from pRPA-BCAT4 (Application FR 96/13077), to produce the plasmid pRPA-BCAT19.
  • the partial sequencing of pRPA-BCAT19 confirmed the replacement of the codon of the Asp279 residue of the nitrilase with the codon of an Asn279 residue.
  • the 1.2 kb EcoRI-XbaI fragment of pRPA-BCAT19 containing the P trp ::RBScII::nitB fusion was then cloned into the vector pRPA-BCAT28 opened with the same enzymes, to produce the 6.2 kb plasmid pRPA-BCAT29.
  • the vector pRPA-BCAT28 was obtained by ligating the 3.9 kb SspI-ScaI fragment of pXL642 (CIP, application Ser. No.
  • An internal 361 bp fragment was amplified by PCR reaction using the primers NitB162, and SR, the matrix pRPA-BCAT41, the Pwo polymerase (Boehringer) and the following incubation program: 5 min at 95° C., five cycles (one minute at 95° C., 1 min at 58° C., 1 min at 72° C.), 35 cycles (45 seconds at 95° C., 30 seconds at 58° C., 30 seconds at 72° C.), 5 min at 72° C.
  • the DNA was incubated in the presence of the NdeI and BanI restriction enzymes (New England Biolabs) at 37° C. for 16 h in a buffer recommended by the supplier.
  • the vector pRPA-BCAT72 was opened with the NdeI and StuI enzymes and the 5.43 kp fragment was purified on agarose gel and extracted using the QIAEX gel extraction kit (Qiagen).
  • the three fragments described above were then ligated and the ligation mixture was introduced into the E. coli strain DH5alpha by electroporation.
  • the clones obtained were analyzed by restriction with the EcoRI and XbaI enzymes in order to select plasmids having a 1.26 kb insert.
  • 2 plasmids carrying the desired codon were selected and named pRPA-BCAT75.
  • the RPA-BIOCAT496 strain corresponds to the W strain (ATCC9637) into which the plasmid pRPA-BCAT34 has previously been introduced.
  • the plasmid pRPA-BCAT34 corresponds to the plasmid pXL2035 (Lévy-Schil et al., 1995, Gene 161: 15-20) into which a 475 bp fragment carrying the trpR gene encoding the regulator of the Ptrp promoter has been cloned between the EcoRI and NotI sites.
  • This fragment was extracted from the plasmid pRPA-BCAT30, constructed by cloning into the vector pSL301 (Brosius, 1989, DNA 8: 759-777) a 434 bp AatII-StuI fragment carrying the trpR gene and its promoter extracted from the plasmid pRPG9 (Gunsalus and Yanofsky, 1980, Proc. Natl. Acad. Sci. USA 77: 7117-7121).
  • the RPA-BIOCAT526, RPA-BIOCAT527 and RPA-BIOCAT497 strains were cultured under the conditions described in example 5 of Application FR 96/13072, with the following modifications: preculturing for 8 h, seeding at 1:50 into M9 glucose medium containing 0.4% of casamino acids, 12 ⁇ g/ml of tetracycline and 50 ⁇ g/ml of kanamycin.
  • the nitrilase activity of HMTBN was measured on a cell pellet washed in 100 mM potassium phosphate buffer, pH 7, as described above, using 1 mg of DC in a reaction volume of 1 ml.
  • the selectivity of the strains was measured after 2 hours of hydrolysis, relating the surface area of the peak of amide formed to the sum of the surface areas of the peaks of amide and of acid formed. It is expressed as a percentage. The results are given in table 1:
  • the codon of glutamine 162 of the NitA nitrilase was modified to a cysteine codon by site-directed mutagenesis using the QuickChangeTM site-directed mutagenesis kit (Stratagene).
  • NitA 163 SEQ ID NO. 4 GCATGTTCCCAGCAGCAGAGTCCCCCAAGATTCC
  • NitA 162 SEQ ID NO. 5 GGAATCTTGGGGGACTCTGCTGCTGGGAACATGC
  • the incubation program for the PCR reaction comprised 30 seconds at 95° C. and 16 cycles of 30 seconds at 95° C.-1 minute at 55° C.-12 minutes at 68° C.
  • the clones obtained were analyzed by plasmid restriction profile using the BpmI enzyme. Since the mutation introduced modifies a BpmI restriction site, five clones which have lost one of the 3 BpmI sites were selected.
  • the plasmids which they contained were introduced, separately, into the RPA-BIOCAT496 strain, to give the strains RPA-BIOCAT570 to RPA-BIOCAT574.
  • the plasmid pXL2158 was introduced into the RPA-BIOCAT496 strain to give the RPA-BIOCAT575 strain.
  • the strains RPA-BIOCAT570 to RPA-BIOCAT575 were cultured under the expression conditions described above, replacing tetracycline with ampicillin at 100 ⁇ g/ml.
  • the nitrilase activity of these strains was assayed as described above, using 5 mg of cells (as dry weight estimated from the OD 660 ) in a reaction volume of 1 ml and for 1 hour.
  • the selectivity is measured by calculating the ratio of the surface area of the peak corresponding to the amide to the surface area of the peak corresponding to the acid. It is expressed as a percentage. Table 2 gives the results.
  • the RPA-BIOCAT570 and RPA-BIOCAT575 strains were cultured under the expression conditions described above. The nitrilase activity of these strains was assayed using 2-amino-4-methylthiobutanenitrile or AMTBN (C5H10N2S) as substrate, at 50 mM in borate buffer, pH 9.2. The equivalent of 10 mg of RPA-BIOCAT570 cells (expressed as dry weight) and of 0.4 mg of RPA-BIOCAT575 cells (expressed as dry weight) were used at 30° C. for 24 h in a reaction volume of 1 ml. The production of AMTBS and AMTBA were measured by HPLC as described above and the selectivity was calculated in a similar manner. The results are given in table 3.
  • the RPA-BIOCAT530, RPA-BIOCAT531 and RPA-BIOCAT497 strains were cultured under the conditions described in example 3.
  • the nitrilase activity on HMTBN was measured on a cell pellet washed in 100 mM potassium phosphate buffer, pH 7, as described in example 4 of Application FR 96/13072, using 1 mg of DC in a reaction volume of 1 ml.
  • the selectivity of the strains was measured after 2 hours of hydrolysis, relating the molar amount of amide formed to the sum of the molar amounts of amide and of acid formed. It is expressed as a percentage.
  • Table 4 The results are given in Table 4:

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